PROJECT SUMMARY E-cigarette use among U.S. youth is a significant public health problem. Understanding youth appeal of e- cigarettes and marketing strategies targeting youth is critical to inform the U.S. Food and Drug Administration (FDA)?s goal to prevent youth e-cigarette use. A significant youth appeal is the ability to modify e-cigarettes or to ?hack? the devices or e-liquid constituents for unintended purposes (e.g., using at higher temperatures or for a longer period of time, adding their own e-liquids made with nicotine or THC oil). However, it is largely unknown how youth modify e-cigarette devices or their components, their perceptions of these behaviors, and marketing strategies used to endorse these behaviors. YouTube, a popular social media, can be examined to understand modified uses of e-cigarettes and marketing associated with these behaviors. The overall goal of this study is to understand ways in which youth modify e-cigarettes, their motivations for doing so, and marketing sources, using YouTube analysis, survey methodology, and expert evaluation. We will identify modified uses of e-cigarettes and marketing using fictitious youth YouTube viewer profiles to search for e-cigarettes using a browser plug-in and custom scripted web-crawling and then machine learning to automatically code the videos to identify e-cigarette modifications and marketing source. Our Advisory Committee (AC) of subject matter experts in tobacco regulatory science, social media, youth tobacco use, toxicology, communications, and tobacco marketing will apply their knowledge to assess the potential impact of these identified modified uses on appeal, addiction, and health effects of e-cigarettes to identify those which are amenable to FDA regulations and communications. We will then conduct an in-depth analysis of these selected behaviors using a combination of machine learning and detailed human coding on youth-accessible YouTube videos to understand: 1) components that are modified (e.g., batteries, cartridges/pods, e-liquids), 2) motivations for modification (e.g., enhance user experience, reduce perceived harm), 3) marketing sources (e.g., online shops, pro-e-cigarette organizations), and 4) appeal (number of likes, views). We will also use an online survey with adolescent and young adult e-cigarette users (N=1000) to examine the prevalence, appeal, motivations, risk perceptions, and marketing exposure related to these modified uses and their predictors (i.e., demographic variables, past-month e-cigarette use frequency, e-cigarette dependence, other tobacco/substance use, interpersonal and intrapersonal risk factors). The results of this study will provide the FDA with novel evidence to 1) develop regulations to prohibit the manufacturing and marketing of e-cigarette components that can be modified by youth, 2) restrict youth-accessible marketing of e-cigarettes on social media such as YouTube, and 3) develop prevention campaigns to educate youth about the harms of modifying e-cigarettes.